1. Field of the Invention
The present invention relates generally to image processing, and in particular, relates to interactively registering ultrasound and magnetic resonance images.
2. Background Information
Accurate guidance and localization of the surgical tool within the brain is essential for the success of various neurosurgical procedures, such as biopsy or tumor resection. In addition, minimum interference of the surgical tool with healthy brain tissues reduces the risk of postoperative complications for the patient. The location of the brain within the skull and its dense nature prevent the direct visualization of the surgical tool and the associated structures. To address these problems in neurosurgery, stereotactic systems have been introduced.
Stereotactic systems provide guidance to the surgeon based on preoperative tomographic images, such as computed tomography (CT) and magnetic resonance (MR) images. The first stereotactic systems were based on specially designed frames (called xe2x80x9cstereotactic framesxe2x80x9d) that were attached to the patient""s head both during the preoperative image scan and during the surgery. These stereotactic frames have an inherent three-dimensional (3D) coordinate system, which is associated, through a coordinate transformation, with the preoperative image coordinate system. Based on the preoperative images, surgeons select the target and the surgical path, and refer to the coordinate system of the stereotactic frame to perform the craniotomy and surgery. Stereotactic frames provide high accuracy, but they have several disadvantages:
They are bulky and interfere with the surgical procedure;
Surgical path planning and target localization in the stereotactic frame coordinates are time-consuming and tedious;
There is no real-time feedback on the preoperative images; and
They are invasive.
With advances in sensing and computing technologies, a new generation of frameless stereotactic systems has been developed. These systems use a position sensor (usually optical) to interactively track the position of the surgical tool during the course of surgery. Interactive display of the preoperative images showing the location of the surgical tool provides the surgeon with real-time feedback. Frameless stereotactic systems are easier to use compared to the frame-based stereotactic systems. In addition, there is no bulky equipment involved. Depending on the method used for registering image and physical (e.g., surgical) space, they can be minimally invasive or even non-invasive. A main limitation associated with both frame-based and frameless stereotactic systems is that the intraoperative surgical guidance is based on preoperative images. Thus, if the brain shifts with respect to the skull or deforms during surgery, the guidance becomes inaccurate. Brain shifts or deformations can be caused by surgical manipulations or cerebrospinal fluid flowing out after the craniotomy.
Intraoperative brain imaging has been an alternative solution in providing the surgeon with visual feedback during surgery. Several imaging modalities have been used intraoperatively. These include CT, MR, and ultrasound (US). Intraoperative US has been widely used compared to intraoperative CT and MR imaging because it is: (1) relatively safe (non-ionizing radiation is used), (2) relatively inexpensive, (3) reasonably easy to use in the operating room, and (4) provides a high update rate. However, the problem with US imaging is its low signal-to-noise ratio (SNR) due to the physics associated with the formation of US images. Moreover, US images contain errors associated with the variation of the speed of sound in different media and the low resolution in the axis perpendicular to the US plane (e.g., azimuthal resolution). Therefore, accurate target localization cannot be solely based on US images. When high accuracy is needed, stereotactic systems currently provide an available option, provided that no brain shifts and deformations occur intraoperatively. Finally, the numerous positions and orientations of the US image planes with respect to the skull, combined with their low SNR, make it difficult for the neurosurgeons to interpret the intraoperative US images and associate them with known brain structures.
Recently, several researchers have tried to integrate intraoperative US images with stereotactic systems. The motivation behind this approach is to combine the real-time intraoperative information contained in US images with the rich anatomical content of preoperative MR/CT images. The main approach for performing this integration has been to use the patient""s skull as a reference in order to register each two-dimensional (2D) US image with the preoperative 3D MR/CT images, where a position sensor is used to track the position and orientation of the US probe in 3D space. An articulated arm, an optical position sensor and an ultrasonic sensor have been used. This US-MR/CT registration enables reconstruction of the 2D preoperative MR/CT images of the brain with the same position, orientation and scaling as the intraoperative 2D US images.
Although comparison of these corresponding US and MR/CT images provides the surgeon with (i) better assessment of the orientation and content of the intraoperative US images and (ii) easier visualization of the intraoperative changes in the brain, the existing integration methods of US systems with stereotactic systems suffer from a number of drawbacks. For example, the optical position sensors require a line of sight. Further, the equipment used by existing methods is cumbersome, complex, and expensive.
Accordingly, there is a need for improved methods of registering US and MR images.
According to one aspect of the invention, a method registers a coordinate space associated with images of a first modality to a coordinate space of a magnetic position sensor, to obtain a first transformation. A coordinate space associated with images of a second modality is registered to the coordinate space of the magnetic position sensor, to obtain a second transformation. The method converts coordinates of images associated with one of the modalities to coordinates of images associated with the other one of the modalities based on the first and second transformations.
Another aspect of the invention provides a calibration method.